(1) Field of the Invention
The present invention relates to colorants used in the IT-related machines and instruments.
More particularly, the present invention relates to colorants for color inks for use in the inkjet type printers utilizing the Piezo effect, bubble-jet type printers, etc. which are required to have a low viscosity and especially a low viscosity at high temperatures.
(2) Description of the Related Art
As the colorant of the color inks for use in the inkjet type printers, oily inks constituted of an oil-soluble coloring matter substance and an organic solvent were used at first. Recently, however, water-based inks containing a water-soluble coloring matter substance have been developed according to the needs of making inks aqueous, and are used prevailingly.
However, water-based inks containing a water-soluble coloring matter substance are apt to undergo photo-degradation of the color in printed matter, because the coloring matter substances used therein are low in durability to light. As is well known, the use of pigments having higher light-durability as the coloring matter substance is drawing attentions and a practical use of such pigments is being set forward.
Unlike soluble coloring matter substances that are present as molecules in a medium, the pigments as the coloring matter substance take a state of particles in a medium. Accordingly, the conversion from soluble coloring matter substance to pigments would have not become possible without the development of a dispersing technique of making the particle diameter of the pigment small to such an extent that the state of dispersion can be approximated to that in a solution. This fact is also known in the art.
It is well known that, regardless of the mode of the machine to be used, the inks for inkjet are essentially associated with a technique of transferring a liquid ink containing a chemical substance through a capillary at a high speed. Accordingly, one of the factors controlling the conditions of color is the chemical structure of a coloring matter substance in use. If this factor is excepted, the physico-chemical conditions which the liquid ink must satisfy are viscosity and those derived from the temperature dependency of viscosity. Specifically, said conditions include the viscosity of a medium, which controls the viscosity of the system, and the spatial size of the dissolved or dispersed solute, and the concentration and the temperature-dependence thereof.
The developmental efforts promoted from this point of view is to simultaneously develop a method for dispersing a coloring matter substance into fine particles, and a dispersant that aids the dispersion of a coloring matter substance, which cannot disperse by itself into fine particles, without increasing the viscosity of the system, as an essential ingredient. The examples thereof include a combination of mechanical and chemical dispersing forces (JP-A-2000-119571), a method for chemical production of fine particle dispersion (JP-A-10-298294, JP-A-2000-119141, and JP-A-316242), a technique relating to an amphoteric resin having a dispersing performance (JP-A-2000-026560), etc.
On the other hand, regarding the improvement of a colorant containing a water-soluble coloring matter substance which takes advantage of the water-solubility, a combination of a water-soluble coloring matter substance and an amino acid, which is an amphoteric electrolyte, or the like (JP-A-2001-139854, JP-A-2000-136335, JP-A-9-12944, and JP-A-7-228816) can be referred to.
Further, an attempt to improve the water-resistance of a water-soluble dye type coloring matter substance by combining an oily dye type coloring matter substance, in place of a pigment, with a water-soluble resin dispersant (JP-A-2002-249689 and JP-2002-249687) is also reported.
As above, the development of inkjet inks for making the most of the characteristic features of a coloring matter substance according to the properties of the coloring matter is ceaselessly continued at the present time. Nevertheless, the study has not yet reached the completion of the development, namely the provision of an ink capable of sufficiently fulfilling the required quality.
Reviewing the actual situation, the following can be pointed out. Thus, though pigment type inks are advantageous in terms of durability, they require pulverization into fine particles, thus the use of a high performance, expensive dispersing machine for the manufacture. Further, they require a polymeric dispersant which limits the lowering of viscosity and, as a result, the instruments in which such a ink is suitably usable are also limited (selectivity of instruments).
The method of using an oily dye type coloring matter substance as an aqueous dispersion is not so greatly different from the above-mentioned pigment system, because it requires a dispersing machine and uses a dispersant.
Although the method of improving the light-durability of a water-soluble water-based dye is epoch-making, this method seems to remain unreliable as it essentially uses an photo-degradable coloring matter substance and requires a photodegradation-inhibiting agent.